Intersymbol interference (ISI) is a serious problem in digital communications systems. Post-cursor ISI occurs when an earlier symbol, composed of one or more bits transmitted at a specified symbol rate, interferes with a later symbol. Pre-cursor ISI occurs when a later symbol interferes with an earlier symbol. Serial link communications systems, such as Serializer/Deserializer (SerDes) systems, are particularly susceptible to pre-cursor ISI.
Various ways have been developed to reduce ISI at both the transmitter end and the receiver end of a communications system. Existing methods attempt to equalize transmitted data signals, with the objectives of correcting for the effects of channel attenuation and complete cancellation of ISI. A conventional serial receiver consists of an analog front end that generally includes a continuous time linear equalizer (CTLE), a sampler that quantizes the analog input into digital values, a decision feedback equalizer (DFE) that uses the quantized data to adaptively feedback a correction signal to the input of the receiver, and a timing recovery unit. CTLEs and DFEs are effective at removing post-cursor ISI, but fail to adequately correct pre-cursor ISI.
Some transmitters in serial link systems implement a feed-forward equalizer (FFE) in the transmitter to provide fixed, i.e., non-adaptive, post-cursor ISI cancellation. The FFE in these serial link systems does not provide any adaptive pre-cursor ISI correction. It is possible to implement an FFE in the receiver. In fact, some receivers include a discrete time FFE that is implemented in the analog or digital domain. Such receivers can handle both pre-cursor and post-cursor ISI, but are structurally complex and consume a large amount of power.